This invention relates to injection molding and more particularly to a method and apparatus which uses stress and flow calculations and closed loop control to monitor and maintain optimum melt pressure of a molten material as it is processed by an injection apparatus.
When molten plastic is processed by an injection molding machine, the plastic enters a mold cavity where it is cooled to form a desired part shape. As the cooling occurs, the plastic contracts within the cavity. As a result of this contraction, the part actually shrinks in size, and sink marks or low spots often occur on the surface of the part. Shrink and sink marks have caused major problems for injection molders since injection molding was first developed. Several methods have been developed in an attempt to eliminate these problems. Some examples include gas-assisted injection molding, structural foam molding, liquid gas assisted molding, etc. In addition, foaming agents have been used in the molding process for mixing with molten plastic in order to generate inert gases in the plastic. These gases provide internal pressure in the plastic which enables the plastic to more fully fill the cavity of the mold and packs the plastic against the cavity walls. This, in turn, helps reduce sink on the surface of the plastic parts. Also, gas counterpressure in the mold cavity has been used to improve surface smoothness of molded parts.
These prior art methods are all problematic due to the large number of variables in the molding process. Varying injection pressures and injection speeds, varying melt pressures and temperatures, varying cavity conditions, and uncontrolled venting of gases all contribute to an unstable molding environment. These various problems in the molding process create burning and scission of polymer chains and create internal stresses within the plastic which remain in the plastic as the plastic material cools in the cavity. These internal stresses cause shrink, sink, and warpage of the plastic part to be molded. In addition, these various molding problems lead to degradation of the plastic material as it is processed through an injection molding machine. In general, erratic variations in pressure, temperature, and injection speed create material break-down and cause internal problems in the plastic which show up in the final molded product.
Another disadvantage of prior art systems is that the plastic melt flow in these systems experiences changes in pressure due to changes in cavity geometry as the molten plastic moves into the cavity of the mold. These pressure changes cause certain areas of the cavity to be filled more quickly than other areas resulting in different cooling characteristics in different areas of the cavity. These cooling variations cause inconsistencies in the direction of plastic solidification, which results in surface stresses, weld lines, or sink.
Gas assisted injection molding is a process for forming a hollow part in which a pressurized assist gas is injected into the molten material either in the nozzle or within the mold cavity. The assist gas forms a hollow bubble within the part which reduces the part weight, thereby reducing material costs. A major problem with this process is that it is very difficult to control the movement of the bubble within the molten plastic in the cavity. Therefore, the hollow part often has walls of uneven thickness.
U.S. Pat. No. 5,558,824 attempted to solve the problem of bubble movement within the mold cavity by prepressurizing the mold cavity with an inert gas, and controlling the release of the gas from the cavity to prevent blow out of the bubble from the interior of the molten plastic. It provides pressure sensors for sensing the pressure of the gas in the mold cavity and the pressure of the gas injected into the bubble.
The primary problem with the disclosure provided in the ""824 patent is that there is no means provided for controlling the pressures acting on the bubble, and therefore bubble formation and movement cannot be controlled. Using this process, Mr. Shah will be unable to control the bubble because it is too difficult to respond in real time by altering the pressures in the cavity and within the bubble by means of valves outside the mold cavity and pressurized gas sources. Furthermore, gas is too compressible to provide the capability of controlling the melt pressure of the molten material in the cavity during injection in real time. Using this system, one cannot recognize mold cavity resistance or resistance from partial solidification, and therefore cannot respond accordingly and control the growth of the bubble. For instance, if the molten material is injected through a very thin area in the cavity, the mold cavity resistance increases, and an increase in bubble pressure will be sensed, but the source of this bubble pressure will not be known, and no means are provided for responding, other than by altering the discharge rate of gas from the cavity, which will be ineffective.
FIGS. 15a-c illustrate the flow of a typical gas assisted melt front through a mold cavity in accordance with the prior art. The melt M includes an assist gas bubble B therein. As the melt front reaches the cavity flow restriction R, as illustrated at FIG. 15b, this flow restriction will cause an increase in the pressure of the melt, which will pressurize the bubble, and will often cause the bubble to blow through the forward surface of the melt front, which would result in a scrapped part. If the melt front were to flow past the restriction, the bubble B will likely move closely adjacent the corner of the restriction R, thus resulting in a very thin section T to be formed in the final part as molded. This thin section greatly reduces the structural integrity of the final part as molded. Also, the material M may become thin at the forward edge of the melt front, and the bubble may blow through this thin portion at any time.
It is desirable to use a balanced injection molding process in which the pressure of the molten plastic is continuously controlled as the plastic moves through the injection molding machine. It is further desirable for an injection molding process to balance pressures acting upon the molten material in order to eliminate the above referenced problems caused by variations in polymer chain conditions so as to reduce internal stresses in the plastic. The ultimate goal of such an injection molding process is to produce a final product which nearly perfectly matches the cavity surface of the mold, is fully relieved of internal stresses which lead to shrink, sink, and warpage, and has greatly improved mechanical properties. In addition, part weight may be reduced, which will provide significant material savings to the manufacturers of such products.
This invention stems from the realization that, when injecting molten material into a mold cavity, it is desirable to preload the system with pressure, which provides conditions under which pressure changes become measurable, and controllable pressure differences may be established between the pressure of gas in the mold cavity and the pressure of the molten material. By providing real time closed loop control, the gas pressure and static melt pressure of the molten material may be sensed and mathematically monitored by the controller in order to provide optimal pressure conditions for injection and solidification of the molten material into the mold cavity. This closed loop pressure control on the basis of pressure differences created from preprogrammed transition of a preloaded melt into a mold cavity provides the capability to control the internal static pressure of the melt throughout the injection and solidification cycle and to provide optimal injection and solidification pressure conditions for the melt as the melt moves from the melt holder and solidifies within the mold cavity.
A method of injection molding is provided for use with an injection molding machine, comprising: (a) generating internal counterpressure within molten plastic as plastic pallets are plasticized in the injection molding machine; and (b) pressurizing air within a cavity of a mold in the injection molding machine to an air pressure level which is substantially equal to the internal counterpressure in order to counterbalance the internal counterpressure as the molten plastic is injected into the cavity, thus providing a substantially pressure-balanced molding environment for the plastic.
Also provided is a method of reducing internal stresses in plastic parts formed in a mold cavity from molten plastic injected into the mold cavity by an injection molding apparatus. The method comprises: (a) pressuring the cavity to a predetermined air pressure; (b) operating the injection molding apparatus to develop molten plastic at a first melt pressure equal to the predetermined air pressure; (c) communicating the molten plastic with the mold cavity when the predetermined air pressure and first melt pressure become equal; and (d) subsequently increasing the melt pressure to a second melt pressure, and maintaining a substantially constant difference between the air pressure in the mold cavity and the second melt pressure during a substantial portion of a predetermined period of time in which the molten plastic is being injected into the mold cavity, whereby to optimize pressure conditions acting upon the molten plastic in a manner to reduce internal stresses in the plastic parts being formed.
The present invention also contemplates a method of injection molding for use with an injection molding machine including a mold with a cavity formed therein for receiving molten plastic and a hydraulic unit for creating an injection pressure to fill the mold cavity with molten plastic at a predetermined melt pressure. The method comprises: (a) supplying air to the cavity at a predetermined air pressure; (b) sensing the melt pressure during injection; (c) sensing the air pressure in the cavity during injection; and (d) providing a closed loop controller to monitor the sensed melt pressure and sensed air pressure and to produce signals to the hydraulic unit for maintaining the melt pressure at desired levels.
The present invention further provides a method of injection molding for use with an injection molding machine including a mold therein. The method comprises: (a) determining a maximum stress to be experienced by plastic as the plastic is processed in the mold; (b) generating counterpressure within the plastic prior to injection of the plastic into the mold, the counterpressure being substantially equal to the determined maximum stress; and (c) maintaining the counterpressure within the plastic at least equal to the determined maximum stress as the plastic is injected into the mold.
The present invention further contemplates a method of reducing internal stresses in plastic parts formed in a mold cavity from molten plastic injected into the mold cavity. The method comprises: (a) pressuring the cavity to a predetermined air pressure; (b) pressuring the melt to a first pressure equal to the predetermined air pressure; (c) communicating the molten plastic to the cavity when the pressures become equal; and (d) subsequently increasing the melt pressure to a second pressure for injection.
Also provided for use with an injection molding machine is a method of injection molding, comprising: (a) calculating a maximum stress to be experienced by a shot of plastic to be molded in the injection molding machine, the stress being a result of a volumetric shrink occurring as the plastic is cooled in a cavity of a mold in the machine; (b) pressurizing a shot of plastic to a first melt pressure as the plastic is plasticized in a barrel of the injection molding machine, the first melt pressure being substantially equal to the calculated maximum stress; (c) pressurizing air within the cavity to an air pressure substantially equal to the first melt pressure; (d) commencing injection of the shot of plastic into the cavity in a laminar flow manner, wherein molten plastic flows into said cavity concentrically with respect to a point at which plastic enters the cavity; (e) increasing the melt pressure on the shot of plastic to a second melt pressure, while maintaining the air pressure within the cavity substantially constant, and maintaining a substantially constant difference between the air pressure within the cavity and the second melt pressure during a substantial portion of a period of time which the shot of plastic is being injected into the cavity; (f) sensing the first and second melt pressures and generating feedback signals indicative thereof; (g) receiving said feedback signals, comparing said feedback signals to reference values, and producing signals for controlling said first and second melt pressures; and (h) returning to step (b).
Further provided is a mold for use in an injection molding machine, comprising a front half and a back half of the mold. The front half includes an aperture formed therethrough for receiving molten plastic from the injection molding machine. The front half and back half cooperate to form a cavity therebetween, and the cavity is in fluid flow communication with the aperture to receive molten plastic therefrom. A plurality of vents are formed in one of the back half and front half, the vents having first and second ends thereof. The first end of each of the plurality of vents is in fluid flow communication with the cavity. The vents are configured according to the following formula to maintain a substantially constant air pressure in the cavity as the cavity is being filled with plastic: A=0.24241*W*T1/(C*P1), where A is a cross-sectional area of a vent, W is discharge of pressurized air through the vent in pounds per second, C is a coefficient of flow, P1 is the air pressure in the cavity in pounds per square inch, and T1 is a temperature in the cavity in degrees Fahrenheit. A channel is formed in one of the back half and the front half, the channel being in fluid flow communication with the second end of each of the plurality of vents for transferring pressurized air into and out of the cavity through the vents. A pair of valves are provided in selective fluid flow communication with the channel. One of the pair of valves is adapted to selectively receive pressurized air from a pneumatic line to provide pressurized air to the channel, and the other of the pair of valves is adapted to selectively allow discharge of pressurized air from the channel. A seal is circumscribed around the cavity and positioned between the front half and back half to prevent discharge of pressurized air from the cavity between the front half and the back half of the mold as the cavity is being filled with molten plastic.
The present invention also provides a method of injection molding non-mixable materials, and a product formed thereby. By xe2x80x9cnon-mixablexe2x80x9d materials, it is meant that the materials are not more than partially soluble within each other and retain their substantial identity when mixed, and the materials do not chemically degrade or decompose each other. By using the methods described above and below, by building a substantial static pressure or internal counterpressure within the molten material (i.e. the mixture of non-mixable materials), the materials will substantially separate and solidify in accordance with their respective modulus. Typically, the material with the highest modulus will begin to solidify first, and the lowest modulus material will be squeezed to the surface of the part to be formed as the high modulus material(s) solidifies and shrinks. The pressure balance between the low modulus, still liquid, material maintains the solidifying high modulus material in the center of the part. Accordingly, a part may be manufactured which has a low modulus material on the surface and a high modulus material in the middle of the part. As best understood at the time of filing this application, this configuration may be changed depending upon the slope and relationship of the modulus curves of the materials, and the temperature inside the mold cavity.
Another aspect of the present invention provides a method of gas assisted injection molding in which the above-described methods are used for controlling formation and movement of the bubble within the molten material in the mold cavity. In a preferred embodiment, bubble formation and movement is controlled by sensing gas pressure in the mold cavity, sensing melt pressure in the injection unit, and controlling injection of the molten material into the mold cavity in accordance with the sensed pressures in order to maintain the internal melt pressure of the molten material in the cavity at desired levels during injection. Various embodiments are contemplated. By monitoring and controlling the internal melt pressure within the mold cavity, one can control bubble formation and movement because the internal melt pressure accounts for any mold cavity resistance or partial solidification within the melt, and the internal melt pressure will balance and control the bubble accordingly. The bubble also provides a source of internal pressure for volumetric deficit compensation.
Accordingly, an object of the present invention is to provide a method of injection molding in which a pressure-balanced molding environment is provided for cooling the molten plastic.
A further object of the present invention is to provide a method of injection molding in which degradation of plastic material is decreased as a result of improved processing controls.
Another object of the present invention is to provide a method of injection molding in which less turbulence is created as the molten plastic is injected into the cavity of a mold.
Yet another object of the present invention is to provide a method of injection molding in which surface stresses in the final product are greatly decreased.
A further object of the present invention is to provide a method of injection molding in which shrink, sink, and warpage of the molded part are reduced.
Another object of the present invention is to provide a method of injection molding in which molten plastic cools and solidifies in a consistent, directional manner.
Still another object of the present invention is to provide a method of injection molding in which processing cycle time is reduced, and part weight is reduced.
A still further object of the present invention is to provide a mold capable of maintaining a desired air pressure within a cavity thereof.
Another object is to provide a method of molding non-mixable materials, wherein internal counterpressure is built up within the combination of molten materials to cause the materials to substantially separate when molded.
A further object of the present invention is to provide a method of gas assisted injection molding in which the formation and movement of the bubble within the molten material is controllable.
These and other features, objects and advantages of the present invention will become apparent upon reading the following description therefor together with reference to the accompanying drawings.